When making prints from photographic negatives, it is desirable to use photographic paper which has a contrast selected to achieve satisfactory tone-reproduction of the original image based upon the contrast of the negative. Papers having higher contrasts, for example, are useful in printing negatives that themselves exhibit low contrasts, so that a satisfactory final print can be achieved. As such, photographic manufacturers offer several grades of photographic paper. In order to avoid the need for separate papers of different grades, "variable contrast" papers possessing the ability to achieve different, selected, contrasts depending upon the wavelength of exposing light have also been employed.
Various methods for obtaining variable, or selective, contrast properties in single photographic elements have been proposed. Early proposals included photographic elements containing two distinct emulsions, such as a high contrast silver chloride emulsion which has been dyed so as to render it sensitive to green light and a low contrast silver bromide or silver chlorobromide emulsion with inherent sensitivity to blue light as disclosed in U.S. Pat. No. 2,202,026. With such a combination of emulsions, a high contrast negative could be exposed through a filter passing only blue light, to which the low contrast emulsion is sensitive, thus producing a print of normal contrast. Alternatively, a low contrast negative could be exposed through a filter passing only green light to which only the high contrast emulsion is sensitive, thus again producing a print of normal contrast.
Use of emulsions of different halide types in order to achieve difference in contrast, however, resulted in complications due to different rates of development of the two emulsions. For negatives which do not fall at either extreme of contrast, and which thus require both emulsions to contribute to the final image in varying amount, the different rates of development made it difficult to judge accurately the adjustment of development time and exposure to optimize the final appearance of a print. Image tone was also found to vary depending upon the relative contribution of each emulsion to the final print.
A second approach to providing a variable contrast system involved the use of a single type of emulsion, but the whole emulsion was not completely dyed. This approach requires the use of a sensitizing dye which itself alters the contrast of the emulsion it is used with. A portion (e.g. less than half) of the emulsion may be dyed with sufficient dye to give the silver halide grains of that portion high sensitivity to a wavelength outside the inherent sensitivity and a different contrast without exceeding the adsorptive capacity of the grains, and that portion then mixed with the undyed portion before coating as described in U.S. Pat. No. 2,384,598. Alternatively, the entire single emulsion may be dyed with a restricted amount (less than that required to impart maximum sensitivity to all of the silver halide in the emulsion) of a sensitizing dye which controls the contrast of the emulsion depending upon the amount of the dye used as described in U.S. Pat. No. 2,280,300. A variable contrast effect is obtained as a result of an uneven grain-to-grain distribution of the restricted amount of dye which apparently results from preferential adsorption of the dye on some of the silver halide grains.
In many conventional single emulsion type variable contrast systems, an inherently blue sensitive, relatively high contrast silver halide emulsion is partially sensitized with a green spectral sensitizing dye which also reduces the contrast of the emulsion for exposures to green light. The emulsion may also be dyed with a blue spectral sensitizing dye to enhance the inherent sensitivity in the blue region. Benzimidazolooxacarbocyanine sensitizing dyes have been found to be effective green sensitizing dyes for single emulsion type variable contrast photographic elements. Benzimidazolooxacarbocyanine dyes are disclosed in U.S. Pat. No. 4,987,063, which is specifically directed towards variable contrast elements, and British Patent Specification 1,390,247, the disclosures of which are incorporated by reference.
An important limitation of the preceding approaches to providing variable contrast elements is that they do not allow convenient means to control the green spectral sensitizing dye grain-to-grain distribution, which determines the low contrast characteristic curve obtained upon exposure in the spectral region absorbed by the dye. Characteristic curves are convenient means for describing the sensitometric properties of photographic materials, and are most commonly presented in the form of "D-log E" curves which plot the output variable density (D) against the logarithm of the input variable exposure (E). Such characteristic curves are discussed in James, The Theory of the Photographic Process 4th, 1977, pages 501-510, the disclosure of which is incorporated by reference.
Improper dye distribution can result in distorted characteristic curves, especially in intermediate exposure regions. The use of photographic elements with distorted low contrast characteristic curves may result in a low quality of prints obtained from many negatives.
Prior approaches to manipulating curve shapes for intermediate exposures include introducing some additional component by means of which the overall contrast is altered. For example, U.S. Pat. No. 2,944,901 discloses adding absorber dyes as a means of controlling contrast. This approach, however, results in fairly large and undesirable speed losses.
Other approaches involve constructing the photographic element such that various independent components determine the shape of different regions of the characteristic curve. U.S. Pat. No. 2,620,727, for example, discloses dividing an emulsion into several portions, wherein one portion remains undyed, one portion is essentially completely dyed, the remaining portions are each dyed with progressively less dye, and all the portions are recombined before coating. By varying the amount of dye, the number of portions, and the relative amount of each portion to the whole, the grain-to-grain distribution of the dye may be controlled and the resulting shape of the low contrast characteristic curve adjusted. The method is, of course, awkward and complicated to manufacture.
U.S. Pat. No. 4,175,967 proposes combining at least two variable contrast sub-systems of different sensitivities into a single element. Each sub-system is sensitized in a different spectral region, by which means the user can independently adjust lower and upper contrasts. With intermediate filter dye layers this method could involve the coating of as many as 30 different layers, making it difficult to manufacture. The product is also difficult to use since regions of the characteristic curve respond to exposure variation in different spectral regions.
It would be desirable to provide an improved method of controlling the grain-to-grain distribution of a variable contrast dye, and thereby adjusting the low contrast characteristic curve of a variable contrast photographic material, which did not involve complex manufacturing and operating procedures. It is a further object of the invention to provide a method by which the low contrast characteristic curve may be adjusted independently of the high contrast characteristic curve.